Acciaio inossidabile effect of gas tungsten arc welded 308 and 409

Acciaio inossidabile
Effect of gas tungsten arc welded 308 and
409 stainless steels on their mechanical
properties
A. F. Miranda Pérez, G.Y. Pérez Medina, F.A. Reyes Valdés, I. Calliari, M. Breda
Stainless steels (SS) in automobile sector were previously incorporated mainly due to their decorative
applications. Nowadays, their functional and specific characteristics make them more required and employed
in this sector. Especially attention from automotive manufacturers has been paid in order to improve the engine
efficiency and reduce weight of the vehicle, stainless steels result to be enabled due to their high strength
mechanical characteristics, energy absorption capability, fatigue and corrosion resistance; besides their
ductility which is traduced to an easy manufacturability. When welding is applied some of their characteristics
may be affected and could decrease their mechanical properties. In attempt to avoid these circumstances,
welding experimental practices must be carried out. In this study plates of 308 austenitic and 409 ferritic
stainless steels were welded by Gas Tungsten Arc Welding process with different current values in order to
get their mechanical properties behavior. Tensile tests were performed, it results that for austenitic stainless
steels welds all failed in the fusion zone presenting ductile behavior; however, for ferritic stainless steels brittle
fracture was observed. The maximum value of hardness for 308 austenitic SS was founded in the base metal,
instead for 409 ferritic SS was reached at the heat affected zone.
This study can be a practical guide in the selection of adequate joining methods in order to determine the more
efficient to use in structural automotive industry.
Keywords: Stainless steel - GTAW - Mechanical properties
INTRODUCTION
Automotive industry development is constantly pursuing
on the conductor’s safety and the weight reduction of the
vehicle. The material selection is primordial in engineering
in order to have an adequate and profitable product, metal
matrix composites, advanced high strength steels, polymers and ceramic composites, and stainless steels are
employed to fulfill these sector requirements. Once, stainless steels usage in automotive sector was ornamental
and simply decorative, wheel covers and trim with minor
amount used for valves and hose clamps [1].
For instance austenitic stainless steels are commonly
seen at chemical industries in tanks or vessel internals, although into furnaces and jet engines were cryogenic temperatures are required, while, ferritic stainless steels are
more employed on pulp and paper industry and refinery [2;
Argelia Fabiola Miranda Perez, Gladys Yerania Pérez Medina,
Felipe Arturo Reyes Valdés
Corporación Mexicana de Investigación en Materiales.
Saltillo, Coahuila. México.
Irene Calliari, Marco Breda
Dipartimento di Ingegneria Industriale, Padova University, Via
Marzolo 9, Padua, Italy
La Metallurgia Italiana - n. 10/2014
3]. This is due to their high temperature resistance, corrosion resistance, fatigue resistance and energy absorption capability. Hence, these properties make them able to
meet requirements for automotive sector [4; 5; 6]. In 2001,
Tseng described that an austenitic stainless steels shows
thermal conductivity lower than carbon steels and higher
thermal expansion [7]. Important considerations must be
taken when welding is applied; microstructural alterations
occurs and mechanical decrement properties take place if
there are not adequate welding parameters. Their microstructure austenitic and ferritic depends on the balance of
promoting elements of each phase [4; 8; 9], thus during
welding, heating and cooling cycles can induces to new
diverse phases such as martensite, and modify their mechanical properties.
In certain austenitic stainless steels welds with a completely austenitic matrix are commonly prone to hot cracking [10], and sometimes in the weld filler material delta
δ-ferrite promoting elements are added in order to minimize this tendency [11]. The welding process usually more
employed to joint this types of materials is the Submerged
Arc Welding (SAW), with the economic disadvantage during the joint thus the high cost of the filler metal. Gas
Tungsten Arc welding (GTAW) process does not require filler metal which means eminent economic advantage. This
work was carried out, in order to have a deeper knowledge
49
Memorie
Base Metal
C
S Mn
P
Si
Cr Ni Mo Cu
V
(wt %)
A 308 0,02 0,012 1.28 0.031 0,46 18.23 8.81 0.08 0.102 0.110
F 409 0,03 0,02 1,00 0,040 1,00 13,5 0,5 -
Tab. 1 - Nominal chemical composition of the
austenitic 308 and ferritic 409 stainless steels base
metal.
A 308
Tab. 1- Composizione chimica nominale del metallo base
on the microstructure changes on welded stainless steels
with GTAW. GTAW process in ferritic and austenitic stainless steels had not reached its fully potential so far; since
there is limited weldability data published literature.
This paper deals with the microstructural characterization
of austenitic 308 and ferritic 409 welded stainless steels
with GTAW. Mechanical testing and microstructural examinations were carried out in order to evaluate the weldments
efficiency. Furthermore, their properties-microstructure
relationship was analyzed and explained.
EXPERIMENTAL DETAILS
The all series experimental include 6 specimens of austenitic 308 (A 308) and 6 specimens of ferritic stainless steel
409 (F 409) sheets plates of 8” x 2.5” x 9/34 and 8” x 2.5”
x 7/24 respectively, with chemical composition shown in
Table 1. Mechanical properties of the both stainless steels
grades are given in Table 2.
Base Metal
Yield
Strength
[Mpa]
A 308
F 409
205MPa
207MPa
Ultimate
Tensile
Strength
[MPa]
515MPa
380MPa
Elongation
[%]
Hardness
[Hv]
40%
20%
185
184
Current
[A]
Voltage
[V]
A1
A2
A3
A4
A5
A6
F1
F2
F3
F4
F5
F6
150
175
200
225
250
275
150
175
200
225
250
275
10
10
10
10
10
10
10
10
10
10
11
11
Welding
speed
[mm/s]
10.2
10.2
11.6
12.7
15.6
14.5
5.2
6
6.4
6.8
6.3
8.7
Heat Input
[J/mm]
147.6
172.2
172.2
177.2
159.9
189.5
289.9
290.7
310
327.1
439.3
345.4
Tab. 3 - Welding parameters using GTAW process in
ferritic 409 and austenitic 308 stainless steels
Tab. 3 - Parametri di saldatura con processo GTAW nelle
acciai inossidabili ferritici 409 e austenitici 308.
6490 LV. The SEM was equipped with an energy dispersive
X-ray spectrometer, used for the chemical analysis of the
welded and heat affected areas. For the microstructural
analysis the software “image pro plus” was employed. The
microhardness test was performed using a load of 500 g
with the FM-300 equipment. The tensile strength of welded joints was evaluated by MTS QT-100 testing machine
using a load of 100kNw, based in the ASTM E8-04 [12]
guideline for subsize samples.
RESULTS AND DISCUSSION
a) Initial base metal microstructure
Tab. 2 - Mechanical properties of the austenitic 308
and ferritic 409 stainless steels base metal
Tab. 2 - Proprietà meccaniche degli acciai inossidabili
austenitici 308 e ferritici 409
For the weldments Argon 100% shielding gas was used to
join the sheets plates for butt joints, in one pass along the
longitudinal direction using manual GTAW process with a
3/8” electrode diameter, the welding parameters are given in Table 3 for each material.
All metallographic specimens were prepared using standard procedures, cross sections for optical metallography
were prepared using standard techniques of polishing; for
A 308 samples were etched (1/3 nitric acid, 1/3 chloride
acid, 1/3 water) instead for the F 409 specimens were
etched with Vilella’s reagent (1 g picric acid, 5 mL HCI and
100 mL ethanol).
The welded joints were then observed through optical
(OM) and scanning electron microscope (SEM) Jeol JSM50
F 409
Base
Metal
The austenitic stainless steel 308 micrograph shown in Figure 1A, is for the base metal which consists in austenitic
matrix with equiaxed austenite grains and twins austenitic
grains as well. Instead for the ferritic stainless steels (Figure 1B), entirely ferritic matrix is showed.
b) Welding characterization
Macrographs for the welding profiles are shown in Figure
2, in order to evidence the different welding zones including, heat affected zone (HAZ) and fusion zone (FZ) for
both steels. In the of the ferritic plate grain coarsening was
revealed, produced by GTAW process [13].
Figure 3 presents the microstructure for the HAZ and FZ
respectively for the austenitic stainless steel, on the HAZ
deformed austenitic grains due to the heat exposed by
welding and some coarse grained austenite (Figure 3A).
In the Figure 3B for the specimen F6 surface discontinuities on the welding in the HAZ was presented, this could
be due to the temperature of the base material previously
deposited weld metal is not elevated to its melting point
during the welding process or for unsuitable welding conditions. Instead in the fusion zone (FZ) austenite dendrites
were observed (Figure 3C).
La Metallurgia Italiana - n. 10/2014
Acciaio inossidabile
Fig. 1 - Optical micrograph for the
transversal section of the a) 308
austenitic stainless steel, b) 409
ferritic stainless steel base metal.
Fig. 1 - Micrografia ottica della sezione
trasversale del metallo base: a) acciaio
inossidabile austenitico 308, b) acciaio
inossidabile ferritico 409.
Fig. 2 - Macrographs of the
welding profile, F1) F 409 and A1)
A 308
Fig. 2 - Macrografie del profilo di
saldatura, F1) F 409 e A1) A 308
Fig. 3 - 308 austenitic stainless steel, a) HAZ in the superior side with deformed austenitic grains, b) Micrograph
evidencing incomplete welding penetration, c) austenite dendrites grains in the fusion zone.
Fig. 3 - Acciaio inossidabile austenitico 308, a) ZTA nella parte superiore con grani austenitici deformati, b) Micrografia che
evidenzia una penetrazione incompleta della saldatura, c) Dendriti di austenite nella zona di fusione
Fig. 4 - Optical micrograph: a) the microstructure is fully ferritic in BM b) ferritic matrix with islands of
martensite at the grain boundaries and c) lath martensite in FZ. 200X
Fig. 4 - Micrografia ottica: a) microstruttura completamente ferritica nel metallo base, b) matrice ferritica con le isole di
martensite ai bordi grano e c) lamelle di martensite nella zona fusa a 200X
The microstructural characterization of the F 409 stainless steels showed in Figure 4, the heat affected zone has
martensite islands at the grain boundaries with a coarse
ferrite matrix, in this case is due to the reversion to austenite when the steel is heated close to liquidus during
welding transform completely to δ-ferrite growths and reversion happens. The HAZ presented in Figure 4B is for
La Metallurgia Italiana - n. 10/2014
the specimen welded with the lower current, it has more
martensite grains formation, which was confirmed by the
microhardness test.
For the fusion zone ferrite matrix is evidence within martensite laths on their grain boundaries, being a typical microstructure for welded ferritic stainless steels.
51
Memorie
Fig. 5 - Macrograph evidencing the indentations for
the microhardness test.
Fig. 5 - Macrografia delle indentazioni ottenute dalla prova
di microdurezza.
c) Microhardness evaluation
Figure 5 evidence the indentations location during the measurement for the microhardness test in a representative
specimen.
In the graph showed in Figure 6 for the weld cross sections,
F 409 ferritic stainless steel representative specimen presents higher hardness in the heat affected zone, this confirms the OM observation were some greater martensite
areas were evidenced. The maximum hardness reached
is 260 HV located at the HAZ. Martensite phase which is
formed in the stainless steel decrease their mechanical
properties as toughness and ductility [12].
In A 308 stainless steel case the measures do not present
variation between the different zones along the weld.
d) Factographic evaluation
For this study were selected 6 specimens (3 of each grade). In the scanning electron micrographs from Figure 7
the fracture for the A308 has a typical dimples which is
characteristic from a ductile fracture it also evidence an
appreciable plastic deformation. On the other hand, brittle
behavior with chevron markings in the F 409 stainless steel
was showed on Figure 8. On more profound observation, it
could be notice that the fracture surface was relatively flat
and there was a lack of appreciable cavitation.
The ultimate tensile strength (UTS) of the as-received stainless steels were 515 MPa for austenitic stainless steel 308
and 380 MPa for ferritic stainless steel 409, the values obtained of the welds drop under from does obtained in the
base metal (365 MPa). Therefore, this evidently suggests
that joining stainless steels by GTAW process conduces to a
reduction in the steel properties specifically toughness and
ductility. In both cases the fracture was initiated on the FZ.
CONCLUSIONS
This work performed detail experiments in order to obtain
deeper information concerning joining stainless steels
using GTAW process. It concludes as follows:
• Microstructural analysis indicates that, for A 308 au52
Fig. 6 - Exhibited microhardness profiles in the various
regions of the welded stainless steels.
Fig. 6 - Profilo di microdurezza nelle varie regioni degli
acciai inossidabili saldati.
Fig. 7 - Scanning Electron micrographs of the A 308
stainless steel
Fig. 7 - Micrografie elettroniche a scansione dell’acciaio
inossidabile A 308
Fig. 8 - Scanning Electron micrographs of the F 409
stainless steels
Fig. 8 - Micrografie elettroniche a scansione dell’acciaio
inossidabile 409 F
stenitic grains on base metal was showed following
coarse austenitic grains in the HAZ, and dendrites
structure at the FZ was highlighted. In the case of F
409, martensitic islands were observed at HAZ which
had an increment on the microhardness values. Martensite laths were found at the FZ. Furthermore, the
martensite grains formed in these zones decrease
their mechanical properties as showed in the factrographic analysis.
• Microhardness test confirms the microstructural observations, martensite phase in F 409 lead to an increment of the hardness particularly in the HAZ.
La Metallurgia Italiana - n. 10/2014
Acciaio inossidabile
• In the tensile shear test values founded at literature
were lower than those obtained at the test. In addition, the parameters used in GTAW welded stainless
steels decrease the steel properties such as toughness and ductility.
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Effetto della saldature ad arco con elettrodo
di tungsteno sulle proprietà meccaniche in acciai
inossidabili 308 e 409
Parole chiave: Acciaio Inox - Saldatura - Prove meccaniche
Nel settore automobilistico, per molti anni gli acciai inossidabili sono stati principalmente utilizzati come parti
decorative. Oggigiorno, invece, le loro caratteristiche funzionali e specifiche li rendono interessanti per impieghi in
questo settore. I produttori di componenti automotive prestano, infatti, particolare attenzione ai materiali che possano
migliorare l’efficienza del motore e ridurre il peso del veicolo e, in tal senso, gli acciai inossidabili rappresentano
una scelta ottimale per questo settore, soprattutto grazie alle loro elevate caratteristiche meccaniche, di tenacità,
di fatica e di resistenza alla corrosione, oltre alla loro duttilità, che si traduce in aumento della lavorabilità. Tuttavia,
quando questi materiali vengono saldati, alcune delle loro caratteristiche possono peggiorare.
In questo studio, si sono analizzati giunti saldati in acciaio inossidabile austenitico AISI 308 e in acciaio inossidabile
ferritico AISI 409, realizzati attraverso processo di saldatura ad arco con elettrodo di tungsteno (Gas Tungsten Arc
Welding – GTAW) e utilizzando differenti valori di corrente al fine di determinarne il loro comportamento dal punto
di vista delle proprietà meccaniche. Le prove di trazione eseguite sui talloni di saldatura hanno evidenziato un
comportamento duttile per gli acciai inossidabili austenitici, mentre per gli acciai inossidabili ferritici si è osservata
rottura di tipo fragile. Il valore massimo di durezza è stato riscontrato nel metallo base per l’austenitico 308 e nella
zona termicamente alterata per il ferritico 409.
Questo studio può essere una guida pratica nella selezione dei metodi di giunzione adeguati al fine di determinare
il più efficace nell’industria automobilistica.
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